A laser beam is used in laser-annealing applications to crystallize an amorphous material to obtain a crystalline, or polycrystalline, material. For example, excimer lasers may be used to crystallize an amorphous silicon (a-Si) film to obtain a polycrystalline silicon (poly-Si) region. In several of these methods, a mask is inserted into the path of the beam to shape the laser beam before the beam irradiates the material.
The mask, used to shape the beam, can in principal have a wide variety of patterns on it. The mask may comprise a patterned layer of chrome, or other material that blocks the desired wavelength effectively, on a quartz substrate, or other suitably transparent material at the wavelength of laser to be used. Common patterns consist of groups of rectangular shapes, including slits and chevrons.
As the laser beam is projected through these patterns, the intensity profile of the projected beam will be determined by the features that make up the pattern and any optics used to image the pattern on the material. The intensity profile of the laser beam is typically not uniform over the entire pattern. For example, at corner regions intensity peaks have been noticed. These intensity peaks may cause local damage on the film irradiated by the shaped beam.) One form of damage caused by intensity peaks is agglomeration, which may cause the silicon film to pull away from the region exposed to the high intensity peaks, possibly leaving a void or other non-uniformity.
Accordingly, a method of suppressing energy spikes is provided comprising projecting a laser beam through a mask having a slit pattern comprising a corner region with edges, and a blocking feature with the corner region to reduce energy spikes projected on a substrate. An alternative method is provided, wherein the corner region is modified such that it is replaced by a more tapered shaped region, preferably a triangle.
Also provided, are a variety of mask designs incorporating both corner regions, with and without one or more blocking features, and triangular regions, with or without one or more blocking features. The mask designs provide examples of mask modifications that may be used to reduce energy spikes.